The present invention relates to a method of inspecting a magnetic disc and an apparatus therefor. More particularly, the present invention relates to a method of inspecting a magnetic disc, which comprises an optically inspecting step and a magnetic inspecting step.
Conventionally, the inspection of magnetic discs has been conducted in such a manner that signals are written thereon and then read out therefrom by a magnetic head, thereby detecting defective portions having a larger change in signal output relative to the average signal output level.
Specifically, by using a magnetic inspection apparatus for magnetic discs as shown in FIG. 2 (generally called "certifier"), given signals are recorded on a magnetic disc 1 and then reproduced therefrom by a magnetic head 2 having a signal processing unit 11 to examine signal output levels, whereby portions from which the signal outputs beyond a certain threshold value are generated, are detected as defects.
However, in association with recent increase in storage capacity and track density of magnetic discs, there arises a problem that the conventional inspection method requires too much time to inspect such magnetic discs, resulting in deterioration in the productivity thereof. Consequently, in order to shorten the inspection time in the magnetic inspection method, there has been proposed a method of inspecting tracks on the magnetic disc in a track-skipping or jumping manner.
However, if the magnetic disc are magnetically inspected in such a track-skipping or jumping manner, there arises such a problem that defects on the tracks located in regions excluded from the inspection can be no longer detected. In the case where the sizes of defects on a magnetic disc are smaller than a predetermined size and the number of the defects is smaller than a predetermined number, the defects on the magnetic disc can be corrected by error correction processing at a disc drive, thereby rendering the magnetic disc usable. However, in the case where such defects whose lengths are larger than a predetermined value are located on the same tracks, the error correction processing is no longer effective. As a result, the magnetic disc having such larger defects must be rejected as a defective disc. The smaller defects are distributed uniformly to some extent on the surface of the magnetic disc. Therefore, even when the tracks of the magnetic disc are inspected by the a track-skipping or jumping manner, the number of the smaller defects can be estimated from an area ratio between inspected regions and skipped or jumped non-inspected regions. However, in the case where the larger defects are located on the same tracks, the number thereof cannot be estimated unlike the smaller defects. In consequence, in order to avoid these problems, optical inspection methods have been used.
As the optical inspection methods, there have been exemplified a light scattering method, a laser Doppler method, a laser interference method or the like. The inspection according to the light scattering method has been conducted in the following manner. As shown in FIG. 3, the inspection apparatus used in this method comprises a light source 3 for irradiating light onto the surface of the magnetic disc 1, a first light-receiving unit 4 for receiving a low-angle scattered light and a second light-receiving unit 6 for receiving a scattered light coming through a shield plate 5 in the mirror reflection direction. By using the inspection apparatus, when the scattered light measured in the mirror reflection direction or vicinities thereof has a predetermined light intensity or higher and the detection is continued for a predetermined period of time or longer and when the low-angle scattered light has a predetermined light intensity or lower, it can be judged that defects are detected at the inspected positions on the magnetic disc. On the other hand, when the low-angle scattered light has a light intensity of more than the predetermined value, it can be judged that dusts or other foreign substances exist on the inspected positions on the magnetic disc, thereby distinguishing the detection of dusts from that of the defects.
However, as a result of the present inventors' studies, it has been found that the defects detected by the optical inspection method have not necessarily been consistent with those detected by the magnetic inspection method. That is, some of the optically detected defects can have a magnetic output level of a predetermined threshold value or higher. Therefore, these optically detected defects cannot be considered to be magnetically defective, though some decrease in magnetic output level is observed by the magnetic inspection. Accordingly, the optical inspection method has a problem of so-called "over-inspection".
As a result of the present inventors' earnest studies, it has been found that by first optically inspecting the surface of magnetic disc to detect defects thereon, and then precisely inspecting smaller area than the optically inspected surface and the area including the optically detected defective portions and vicinities thereof by a magnetic inspection method, the defects on and in the magnetic disc can be detected at a high speed and with a high accuracy without failed inspection and over-inspection. The present invention has been attained on the basis of the finding.